docs: create AISIX Cloud docs for environments, certificates, projection, billing, and resilience

[moonming]

Parent issue: #240

Summary

Document what AISIX Cloud adds on top of AISIX AI Gateway for managed control-plane and data-plane scenarios.

Scope

• docs/cloud/overview.md
• docs/cloud/organizations-and-environments.md
• docs/cloud/gateway-certificates-and-managed-dp.md
• docs/cloud/resource-projection.md
• docs/cloud/cloud-playground.md
• docs/cloud/usage-events-and-billing.md
• docs/cloud/provider-key-rotation.md
• docs/cloud/offline-resilience.md
• docs/cloud/cloud-vs-self-hosted.md

Acceptance Criteria

• Cloud-only concepts are separated cleanly from standalone gateway docs
• Certificate-based managed-DP flow is documented accurately
• Projection, billing, and offline-resilience behavior are explained from the customer's point of view
• Cloud playground limitations are documented honestly

[coderabbitai]

🔗 Related PRs

#8 - docs: update the architecture which is much beautiful [merged]
api7/docs#1452 - docs: restructure API7 API Gateway documentation [merged]
api7/AISIX-Cloud#2 - docs: PRD for Hybrid Cloud AI Gateway Dashboard [merged]
api7/contents#807 - seo: add 'What Is an AI Gateway' learning center article [merged]
api7/api7ee-3-control-plane#2588 - feat: support AI gateway groups in etcd proxy [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!

[moonming]

Closed by the merge of #249.